This invention relates to a changeover device which makes it possible to select one modulator out of two in order to use it in a transmitter.
The choice of the modulator to be employed in a transmitter arises when, for reasons of transmission power and operational safety, two transmitters are coupled in order to transmit one or a number of signals but can also be dissociated in order to transmit independently. In fact, in this type of operation, each of the two transmitters has its own modulator whereas only one modulator is employed in a pair of coupled transmitters. Operational safety requirements, however, make it necessary in the event of failure of one of the two modulators to connect the other modulator automatically into both transmitters.
A known practice consists in making use of a control circuit (also designated as an automatic coupling circuit) in order to perform all the control functions relating to the operation of two associated transmitters. In order to take into account the fact that a failure of one of the transmitters or one of the modulators may be only transient, a control circuit of this type connects a modulator into a transmitter only after a time-delay which may be of the order of several seconds. Under these conditions, if a fault occurs in one modulator, there is a potential danger of a transmission accident which may even give rise to other failures and therefore prevent any resumption of normal transmission when the faulty modulator is replaced by the other modulator after the time-delay. This problem is encountered in particular in television when two "sound" transmitters are coupled.